In the prior art, transmissions could be broadly grouped into two categories. In a first category, which is typically utilized on normal on-the-road machines, the transmission is shifted between a plurality of discrete mechanical gear ranges. In a second type of transmission, known as continuously variable transmissions, there is the ability to provide an infinite number of speed ratios between an input and an output shaft.
One known type of continuously variable transmission utilizes both a mechanical multi-range transmission, and a continuously variable component. In one example of this type of transmission, the input to the mechanical multi-range transmission portion is combined with an input from a hydraulic motor to result in a continuously variable output. This type of transmission is known as a hydro-mechanical transmission. Control algorithms for this type transmission have not been as efficient as would be desirable at shift points.
Hydro-mechanical transmissions provide continuously variable ratios by varying hydraulic motor speed between synchronous shift points of the mechanical mechanism. The most common control strategy used with this transmission is to vary the transmission ratio as load varies to maintain a constant engine speed. While this control strategy permits selection of an engine speed where the engine is efficient, it results in the efficiency of the transmission varying significantly from a maximum efficiency where the motor is at low speeds and most of the power is transmitted mechanically to a minimum efficiency at the synchronous shift points where the motor speed and hydraulic power are at a local maximum.
The present invention is directed to overcoming one or more of the problems set forth above.